1. Field of the Invention
The present invention is directed a system for converting a device generic or base profile that is applicable to a family of color devices into a profile for a specified device, and, more particularly, to a system that uses the base profile to generate color outputs for a device, colorimetrically measures the color of the color outputs and adjusts the profile to more accurately characterize the device.
2. Description of the Related Art
The field of color device analysis can be divided into several different areas:
characterization, calibration, and correction. A device can be corrected to remove the effects of distortion caused by the device. In this operation the behavior of the device is compared to the behavior of an "ideal" device and is adjusted until it becomes ideal. Calibration consists of putting the device into a known state. In calibrating a device, the device is adjusted so that a series of known inputs produces a set of predetermined outputs, called an "aim curve".
Once a device is in a known state, it can be characterized. Of course a device can be characterized even when it has not been calibrated. Characterization consists of describing the actual colorimetric behavior of a device. This description is typically accomplished by measuring the colors of patches produced by known inputs to the device, and then fitting the measurements to some sort of model for the device.
Device characterization is important because it allows people to convert between different device color spaces. For example, consider a person who wishes to scan in a picture, view the picture on a monitor, and then write out the picture on an ink-jet printer. To do this in such a way that the original picture, monitor image, and printed copy look the same, it is necessary to convert between the color spaces defined by the RGB measured by the scanner, the RGB used by the monitor, and CMYK used by the printer.
To facilitate the use of device characterizations, color management systems have been developed by a number of companies. These color management systems use what are called device profiles to describe the colorimetric properties of devices. Each profile contains transformations between the device color space and a profile connection space (PCS). One common PCS is CIELAB, but other CIE spaces such as uvL and XYZ have also been used.
These device profiles are commonly produced in a format developed by the Intercolor Consortium, and described in the document "ICC Profile Format Specification" .COPYRGT.International Color Consortium 1995. This document is incorporated by reference herein. It is also available on the internet at the anonymous ftp site maintained by Silicon Graphics Inc., sgigate.sgi.com in the directory .about.ftp/pub/icc. An ICC device profile contains a transform from the profile connection space to the device space and a transformation from the device space to the PCS. In addition, profiles for output devices contain a simulation transform which maps from the PCS to the PCS and describes how out of gamut colors are mapped into the gamut of the output device before being printed.
Currently there are two ways for an individual user to get a device profile for a given device. The first is to obtain an existing profile. The advantage of this method is that the user, without investing much time or effort, gets a profile which has been prepared by skilled engineers using highly accurate equipment. On the other hand a pre-made profile cannot allow for individual variation between different devices of the same type. This is particularly a problem in the case of devices, such as printing presses or color film writers, where the color production process is inherently unstable.
The second current method of obtaining profiles is for the user to create them from nothing. Creating a profile involves writing a large set of patches at fixed ink values, measuring those patches, and then using an application which makes use of the measurements to create the profile. Typically this process requires measuring an extremely large set of patches. Currently available products require the measurement of anywhere from 200 to 400 individual color patches. The analysis of these measurements also takes a long time on the types of computers which are available to most people. Furthermore, the process can be extremely sensitive to errors in the measurements. Because of the large number of patches which have to be measured, these errors can easily be due to human error, as well as being due to the measurement noise associated with low-cost color measurement instruments. Furthermore, because of the large amount of time it takes to make the required measurements, it is not practical to track process variation by constantly recomputing output profiles.
What is needed is a way of combining the advantages of both approaches by creating a more accurate profile or characterization from an existing profile.
U.S. Pat. No. 4,500,919 by Schreiber describes a method in which an iterative process is used to determine ink lookup table values. In Schreiber's method one computes the CMY values expected for a given set of RGB inputs, prints those values, measures the resulting RGB and "adjusts the LUT entries according to the error in CMY" (13,31). However it is not clear from Schreiber's description how the LUT is to be adjusted or how this adjustment effects the handling of out of gamut colors described by the simulation transform. U.S. Pat. No. 4,658,286 to Schwartz describes a system similar to Schreiber's in that adjustments are made to an output to correct measured distortions caused by the device. However, Schwartz does not say anything about how to modify the simulation transform or how to deal with the fact that changes in the colorimetry of an output device will result in changes in the output gamut for the device.
What is needed is a system that will consider the changes in the gamut of a device.